A slide member such as a slide bearing typically used in automobile applications generally comprises a back metal layer coated with Al or Cu based alloy layer. Normally, an overlay is further provided on the surface of the alloy layer to improve bearing properties.
Recent growing increase in engine output has caused slide bearings to be subject to greater surface pressure, and increasing use of low viscosity lubricant oil for achieving improved energy efficiency is causing oil film ruptures and seizures on a more frequent basis. Given such circumstances, there is a need in the industry for a slide bearing with outstanding anti-seizure properties.
One example of a slide member with improved anti-seizure properties is disclosed in patent publication JP 10-77968 A. The disclosed slide member has an overlay formed by electrical Sn plating on the surface of an Al—Si eutectic alloy base material. The maximum surface roughness (Rz) of the Al—Si eutectic alloy base material is maintained at 5.5 μm or less. By keeping the maximum surface roughness (Rz) of the Al—Si eutectic alloy base material at 5.5 μm or less, less bumps are observed on the surface of Sn plating, thus scoring and seizure can be prevented even when sliding takes place under EHL (Elasto-Hydrodynamic Lubrication) with relatively less lubricant oil and higher surface pressure.
The Al—Si eutectic alloy base material generally comprises primary crystal Si particles having relatively large particle size and fine eutectic Si particles. In JP 10-77968 A, the surface of the Al—Si eutectic alloy base material is etched by alkali etching and desmutted prior to being electroplated with Sn. Alkali etching is performed by applying an alkali liquid comprising high concentration of sodium hydroxide solution on the surface of Al—Si eutectic alloy base material while desmutting applies desmutting liquid comprising low concentration of nitric acid etc. Alkali etching and desmutting dissolve primary crystal Si particles and fine eutectic Si particles to reduce the size of primary crystal Si particles and remove most of fine eutectic Si particles. Thus, by reducing the size of or removing Si residing on the surface of the Al—Si eutectic alloy base material, the bumps on the surface of the Al—Si eutectic alloy base material and consequently the bumps on the surface of the overlay are reduced in size.
When bumps, which may also be described as a combination of convexities and concavities are formed on the surface of the overlay, the concavities (or recesses) between the convexities retain lubricant oil to provide improved anti-seizure properties. In employing such approach of retaining lubricant oil between the convexities on the surface of the overlay, distribution and size of the convexities situated in the area in which lubricant is retained, or the so called the lubricant retention area, is considered as an important factor in determining the favorable or unfavorable level of non-seizing properties.
JP 10-77968 A, however, does not address the distribution and size of the convexities of the overlay nor the Al—Si eutectic alloy base material which significantly influences the shape of the convexities residing on the surface of the overlay. As can be seen particularly in JP 10-77968 A, since most of the fine or small sized eutectic Si particles are dissolved by desmutting, the remaining primary crystal Si particles tend to be too far apart from one another. Such excessive spacing between the Si particles on the surface of the Al—Si eutectic alloy base material also results in excessive spacing between the convexities on the overlay surface. Such excessive spaciousness provides grounds for unwanted narrowing of gap between the concavities (between the convexities) and the counterpart slide member when either or both the counterpart slide member and the overlay are deformed (one form of deformation may be a bend). Since lubricant oil is retained in the aforementioned gap, narrower gap results in poor oil retention.
Primary Si particles, initially sizable, are dissolved by desmutting and thus, reduced in size to some extent. However, since the primary Si particles still remain relatively sizable, the convexities produced on the overlay remain relatively sizable as well. Such oversized convexities may prevent smooth lubricant oil flow within the lubricant oil retention area by blocking the flow of lubricant oil into locations in short supply of lubricant oil caused by local contact etc. Such shortage in lubricant oil supply may ultimately lead to seizures.